(1) Field of the Invention
This invention relates to an optical semiconductor device for use with optical communication and a fabrication method therefor, and more particularly to an optical semiconductor device suitable for use, for example, with a semiconductor optical amplifier wherein quantum dots are used for an active layer and a fabrication method for the optical semiconductor device.
(2) Description of the Related Art
In recent years, an optical semiconductor amplifier (hereinafter referred to as quantum dot SOA) having an active layer formed from quantum dots has a broad gain bandwidth and is expected promising as a device which can amplify CWDM (Coarse Wavelength Division Multiplexing) signals.
In order to place a quantum dot SOA into practical use, it is demanded not only to expand the gain bandwidth but also to reduce the polarization dependency and finally achieve polarization independent.
For example, if an active layer is formed on a face parallel to a substrate in a quantum dot SOA, then since the dots have a flattened shape, the gain with regard to TE polarized light is high and a high polarization dependency is exhibited.
Therefore, as a structure for reducing the polarization dependency, an optical semiconductor device wherein quantum dots are formed, for example, on an inclined surface has been proposed (refer to, for example, Japanese Patent Laid-Open No. 2003-204121).
Meanwhile, a semiconductor laser formed taking the polarization dependency into consideration is disclosed, for example, in Japanese Patent Laid-Open No. Hei 10-41587.